Finish PyCallable -> PythonCallable conversion.

This finishes the effort to port python-wrapper.swig code over to
using PythonDataObjects.

Also included in this patch is the removal of `PyCallable` from
`python-wrapper.swig`, as it is no longer used after having been
replaced by `PythonCallable` everywhere.

There might be additional cleanup as followup patches, but it should
be all fairly simple and minor.

llvm-svn: 252939